This invention relates to a process for controlling the molten metal level in continuous thin slab casting and more particularly to a process for controlling the molten metal level in continuous thin slab casting by regulating the amount of pouring metal in accordance with changes in the molten metal level.
In continuous casting, recent manufacturing trends are to produce small-sized thin slabs rather than large-sized ones. However, in order to cast such thin slabs, since not only the cross-sectional area of the slab is small but also the ratio of the thickness to the width is small, the molten metal level during casting greatly varies due to even slight fluctuations in the casting conditions. Also, since high speed casting is required for higher productivity, the control of the molten metal level must be highly responsive even with respect to a large fluctuation of the molten metal level as discussed above.
In conventional large-sized continuous slab casting as shown in FIG. 1, a flow of molten steel 1 is supplied from a ladle 2 into a tundish 3 and then from the tundish 3 into a mold 4. The molten metal level 5 is measured by a suitable detector means 6, which generates a deviation signal representative of the deviation of the measured value from a target value when the casting conditions are changed. The deviation signal is supplied to a regulator 7 which controls a hydraulic servo-valve mechanism 8 to regulate the degree of opening of the valve of the sliding nozzle 9 of the tundish 3. In this manner, the casting flow rate of the molten metal flow 1 from the tundish 3 is regulated to control the molten metal level 5.
The inventors of the present invention have previously proposed a process for continuous thin slab casting, which may be called a three-step metal pouring method, in which the molten metal poured from a large-sized tundish into a small-sized tundish through a sliding valve nozzle is caused to overflow from the small-sized tundish to be poured into a belt-type continuous casting apparatus through a casting spout, whereby a cast slab can be pulled by the movement of the belt.
If the conventional molten metal level control technique as previously discussed in conjunction with FIG. 1 is to be applied to the continuous thin slab casting method described above, the first measure would be to measure the metal level in the casting mold and to regulate the degree of opening of a nozzle valve on the outlet side of the large-sized tundish according to the amount of deviation of the measured value from the target value, thereby controlling the flow rate of the molten metal. However, according to experiments conducted by the inventors of the present invention, the time delay in the change in the molten metal level after a change in the degree of opening of the valve of the sliding nozzle of a large-sized tundish is extremely long; while the time delay is on the order of 0.1 to 0.3 seconds in the conventional method shown in FIG. 1, the time delay in the above-described three-step metal pouring method would be at least ten times as large. Thus, it was determined that as long as the conventional, simple control method is utilized, the accuracy of control is poor no matter how the control gain of the regulator is adjusted, making stable operation almost impossible.
Also, according to the experimental results obtained by the inventors of the present invention with the simple metal level control method utilizing the opening degree of the sliding nozzle, when a disturbance consisting of a sudden expansion or contraction of the nozzle cross-sectional area by 15% is experienced, the molten metal level is changed by at least 17 mm, while the required accuracy is .+-.3 mm. It has also been determined that, where a thin slab having a small thickness compared to the width is cast at a high speed as is done in belt-type continuous casting, the rapid lowering of the metal level in the mold causes the metal level to go out of control, and in an extreme case, the mold may be emptied, making continuous casting impossible. Alternatively, the molten metal may overflow from the mold, creating a very dangerous situation.